The present invention relates to a system for controlling downhole well tools to produce hydrocarbons from a wellbore. More particularly, the invention relates to a back pressure control system providing safe operation in multiple hydraulic control lines.
Downhole well tools control, select and regulate the production of hydrocarbon fluids and other fluids produced downhole from subterranean formations. Downhole well tools such as sliding sleeves, sliding side doors, interval control lines, safety valves, lubricator valves, chemical injection subs, and gas lift valves are representative examples of such tools. Well tools are typically controlled and powered from the wellbore surface by pressurizing hydraulic lines which extend from a Christmas Tree or other wellhead and into the wellbore lower end.
Dual pressure barriers in hydraulic lines are preferred to prevent hydraulic line failure during a wellbore catastrophic event. Dual pressure barrier systems have an active and a passive barrier. The active barrier typically comprises a valve located at the Christmas Tree or wellhead, and the passive barrier typically comprises a check valve located in the hydraulic line below the wellhead. The check valve restricts fluid flow in one direction as the hydraulic fluid, chemicals or other fluids are pumped downhole into the hydraulic line. The fluids pressurize an actuator in a single operation or are discharged into the tubing or wellbore annulus through an exit port or valve.
Certain tools such as safety valves require fluid flow control in opposite directions. However, safety valves do not internally provide dual barrier capabilities because such barriers would resist two-way fluid flow. Because safety valves do not provide a passive well control barrier, significant design effort has been made to enhance the reliability of safety valve operation. Safety valves have been designed with metal-to-metal fittings, metal dynamic seals, rod piston actuators, and other features designed to provide reliable operation during a catastrophic event in the wellbore. Other safety valves use springs, annulus fluid pressure, or tubing fluid pressure to provide the restoring force necessary to return the closure mechanism to the original position.
Downhole well tool actuators generally comprise short term or long term devices. Short term devices include one shot tools and tools having limited operating cycles. Hydraulically operated systems have mechanical mechanisms with simple shear pins or complex mechanisms performing over multiple cycles. Actuation signals are provided through mechanical, direct pressure, pressure pulsing, electromagnetic, and other mechanisms. The control mechanism may involve simple mechanics, fluid logic controls, timers, or electronics. Motive force can be provided through springs, differential pressure, hydrostatic pressure, or locally generated mechanisms. Long term devices provide virtually unlimited operating cycles and are designed for operation through the well producing life. One long term device provides a fail safe operating capabilities which closes with spring powered force when the hydraulic line pressure is lost. Combination electrical and hydraulic powered systems have been developed for downhole use.
Control for a downhole tool can be provided by connecting a single hydraulic line to a tool such as an internal control valve (xe2x80x9cICVxe2x80x9d) or a lubricator valve, and by discharging hydraulic fluid from the line end into the wellbore. This technique has several limitations as the hydraulic fluid exits the wellbore because of differential pressures between the hydraulic line and the wellbore. The discharge of hydraulic fluid into the wellbore comprises an undesirable environmental discharge, and the fluid discharge risks backflow and particulate contamination in the hydraulic system. Additionally, the setting depths are limited by the maximum pressure that a pressure relief valve can hold between the differential pressure between the control line pressure and the production tubing. All of these limitations effectively restrict single line hydraulic systems to relatively low differential pressure applications such as lubricator valves and sliding sleeves.
To overcome these limitations, a second hydraulic line can be installed to return hydraulic fluid to the wellbore surface through a closed loop. In U.S. Pat. No. 4,942,926 to Lessi (1990), dual hydraulic lines provided tool operation in two directions. In U.S. Pat. No. 3,906,726 to Jameson (1975), a manual control disable valve and a manual choke control valve controlled hydraulic fluid flow on either side of a piston head. In U.S. Pat. No. 4,197,879 to Young (1980) and in U.S. Pat. No. 4,368,871 to Young (1983), two hydraulic lines controlled a lubricator valve during well test operations. In all of these tools, two hydraulic lines are inefficient because the additional hydraulic lines increase sealing problems and reduce the available space through packers and wellheads. Additionally, passive barrier protection for each hydraulic line is not possible because of the return fluid flow from the well tool to the surface.
Accordingly, a need exists for an improved system capable of providing back pressure control in systems having multiple hydraulic lines. The system should be reliable, adaptable to different tool configurations and combinations, and should provide passive back flow containment for downhole well tools.
The present invention provides an apparatus for providing back pressure control in at least two hydraulic lines extending downhole in a wellbore. The apparatus comprises a check valve engaged with each of the hydraulic lines in a closed initial position, wherein each of said check valves prevents pressurized fluid downhole of the check valves from moving upstream of the check valves, and hydraulic means operable with the fluid pressure in a hydraulic line to selectively open a check valve engaged with another of the hydraulic lines to permit two-way fluid communication through the check valve. The hydraulic means is further operable when the hydraulic line fluid pressure is reduced to return the check valve to the initial position.
In other embodiments of the invention, each check valve can comprise a pilot operated check valve, and the invention is applicable to three or more hydraulic lines. The hydraulic means can comprise a control valve or control valve combination having fewer valves than hydraulic lines.
In another embodiment of the invention, the apparatus can selectively open fluid flow through hydraulic lines extending between a wellbore surface and a downhole tool. The apparatus can comprise a check valve engaged with each hydraulic line in a closed initial position where each of the check valves prevents pressurized fluid downhole of the check valve from moving upstream of said check valve, a hydraulic means operable with the fluid pressure in a hydraulic line to selectively open a check valve engaged with another hydraulic line to permit two-way fluid communication through the check valve, and a controller engaged with the hydraulic lines for selectively pressurizing at least one of the hydraulic lines to operate said hydraulic means and to open a check valve engaged with another of the hydraulic lines.